1. Field of the Invention
The present invention relates to an electric system for a fuel cell, a fuel cell vehicle, and a method of supplying electric power, and more particularly to an electric system having an electric storage device whose voltage is lower than the output voltage of a fuel cell and a DC-to-DC converter for performing bidirectional voltage conversion between a power supply line connected to the fuel cell and a power supply line connected to the electric storage device, a fuel cell vehicle incorporating such an electric system, and a method of supplying electric power in such an electric system.
2. Description of the Related Art
Recently, fuel cell vehicles carrying fuel cells, which are of excellent fuel efficiency and environment-friendly nature, as propulsive power sources have been developed and put to practical use. Fuel cells for use on fuel cell vehicles are often high-voltage fuel cells because they are required to generate large electric power to provide sufficient drive forces to propel the fuel cell vehicles.
Some fuel cell vehicles also carry electric storage devices for assisting in supplying electric power to meet high loads and load variations and also for storing regenerated electric power.
If the voltage generated by a fuel cell and the rated voltage of an electric storage device are different from each other, then a DC-to-DC converter for performing bidirectional voltage conversion is connected between a power supply line connected to the fuel cell and a power supply line connected to the electric storage device. The DC-to-DC converter allows electric power to be efficiently transferred between the fuel cell and the electric storage device and also allows regenerated electric power to be efficiently stored in the electric storage device.
Fuel cell vehicles incorporate fuel cell accessories for operating fuel cells, i.e., an air compressor, a hydrogen supply pump, a coolant circulation pump, etc., in addition to ordinary motor vehicle accessories including an air conditioner, windshield wipers, power window motors, etc. mounted on general motor vehicles.
One type of connections used for supplying electric power to motor vehicle accessories and fuel cell accessories on fuel cell vehicles is disclosed in Japanese Laid-Open Patent Publication No. 2004-193063. As shown in FIG. 16 of the accompanying drawings, the disclosed system has a power supply line 3 interconnecting a fuel cell 1 and an inverter 2, and power supply lines are branched from the power supply line 3 for supplying electric power to a motor vehicle accessory 4 and a fuel cell accessory 5. The power supply line 3 carries a high voltage suitable for energizing an electric motor. The electric power from the power supply line 3 is reduced in voltage by a DC-to-DC converter 7 before it is supplied to an electric storage device 6. The DC-to-DC converter 7 is capable of converting voltages bidirectionally.
Japanese Laid-Open Patent Publication No. 2002-118981 discloses another system of connections as shown in FIG. 17 of the accompanying drawings. As shown in FIG. 17, a power supply line 8 interconnects an electric storage device 6 and a DC-to-DC converter 7, and power supply lines are branched from the power supply line 8 for supplying electric power to a motor vehicle accessory 4 and a fuel cell accessory 5. The power supply line 8 is of a low voltage setting because the voltage carried thereby is reduced by the DC-to-DC converter to a value lower than the voltage carried by a power supply line 3 interconnecting a fuel cell 1 and an inverter 2.
As described above, many fuel cells mounted on fuel cell vehicles are of the high-output type, and fuel cell accessories for operating fuel cells should preferably be of the high-output, high-voltage type that is capable of withstanding high loads.
Windshield wipers, power window motors, etc. as vehicle accessories are not necessarily required to be of the high-voltage type, but may be of the general-purpose, low-voltage type (e.g., 12V type) for thereby allowing fuel cell electric systems to be constructed inexpensively.
In the systems disclosed in Japanese Laid-Open Patent Publication No. 2004-193063 and Japanese Laid-Open Patent Publication No. 2002-118981, since both the vehicle accessory and the fuel cell accessory are supplied with electric power from the same power supply line, they need to be dedicated accessories compatible with the rated voltage of the power supply line, or a DC-to-DC converter needs to be connected to at least one of the vehicle accessory and the fuel cell accessory, making the electric system complex.
If a complex system including many accessories is constructed based on the system revealed in Japanese Laid-Open Patent Publication No. 2004-193063, then each of the accessories is connected to the high-voltage power supply line 3 interconnecting the fuel cell and the inverter (see FIG. 16). Therefore, these accessories are dedicated accessories compatible with the voltage according to the specifications of the fuel cell or the electric motor, and are hence not for general use. Particularly, as electric accessories on other vehicles such as general engine-driven vehicles or hybrid vehicles cannot be used as those accessories, the complex system is highly costly to construct.
If accessories are to be added to the system disclosed in Japanese Laid-Open Patent Publication No. 2002-118981, then they are connected to the low-voltage power supply line 8 that is connected to the electric storage device 6 shown in FIG. 17. However, the system with the added accessories is disadvantageous in that if the DC-to-DC converter fails and the electric power generated by the fuel cell cannot be supplied to the low-voltage power supply line 8, then the distance that the fuel cell vehicle is able to travel subsequently will be shortened. Specifically, since the fuel cell needs to be supplied with a reactive gas for its operation, it is necessary to operate an air compressor, pumps, etc. for operating the fuel cell. However, as these accessories are connected to the low-voltage power supply line, they are energized by only the electric power which remains in the electric storage device. Consequently, even if a sufficient amount of hydrogen gas is stored in the fuel tank, since no electric power is obtained from the stored hydrogen gas for actuating the air compressor and the pumps, the fuel cell is shut off at the time the discharge of electric power from the electric storage device is finished. In addition, inasmuch as the air compressor and the pumps consume a relatively large amount of electric power, the electric storage device discharges the stored electric power at a high rate and cannot be operated continuously for a long period of time in the event of a failure of the DC-to-DC converter.
If the accessories connected to the low-voltage power supply line 8 in the system disclosed in Japanese Laid-Open Patent Publication No. 2002-118981 are regarded as belonging to a single auxiliary electric system, then it is different from the electric systems on vehicles such as general engine-driven vehicles or hybrid vehicles in that the electric system includes an air conditioner and a pump. Therefore, the electric system is not for general use and is highly costly to construct as with the electric system disclosed in Japanese Laid-Open Patent Publication No. 2004-193063.